Multiplexer
A multiplexer (mux) selects one of multiple inputs to forward to its output. Digital muxes (74HC151 8:1, 74HC153 dual 4:1) handle logic levels; analog muxes (CD4051 8:1, ADG708 8:1) pass continuous voltages and signals up to a few hundred MHz with careful design.
Inside, an array of CMOS pass-gates is steered by a small decoder that reads the select bits. Only the gate corresponding to the selected channel is closed; all other channels are open-circuit (high-Z). For analog muxes, both NMOS and PMOS pass-gates are paralleled per channel so signals can swing rail-to-rail with low on-resistance.
In plain terms
A train station's platform-switching lever: the tracks all converge on one outbound rail, and the dispatcher's lever decides which incoming train gets to leave next.
Why designers use it
- Share one ADC input across many sensor channels to save IC count and PCB area.
- Route different debug signals to one logic-analyser pin under software control during bring-up.
- Combine outputs of several oscillators into one selectable clock source.
- Pick one of multiple audio sources (line-in, mic, USB) in a small DSP-board mixer.
Best for
- ADC sharing
- Clock select
- Audio routing
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- For high-speed (>100 MHz) digital signals where mux on-resistance and parasitic capacitance distort edges — reach for a CMOS crosspoint switch or LVDS-rated mux.
- For very high precision (±0.01 %) DC measurements — leakage currents flowing through neighbouring channels' pass-gates introduce errors; use a dedicated reed-relay matrix.
Common mistakes
- Driving an analog pin beyond the selected mux's recommended signal range, including while its supply is off. Check input limits, sequencing and any allowed injection current.
- Assuming every analog mux passes a signal accurately up to its supply rails under all loading conditions.
- Ignoring on-resistance, leakage, charge injection and settling time in a measurement path.
Where you will find it
- A modular synthesiser's input section uses a CD4051 8-channel analog mux to select between line-in, microphone, instrument, and four CV-jack signals: the same 16-bit ADC samples whichever input is selected via the front-panel rotary, eliminating seven separate ADCs and seven preamps from the BOM.
- A multi-lead ECG electrode harness uses an ADG726 16:1 mux to scan electrode pairs sequentially through a single instrumentation amplifier: the 12-lead ECG is reconstructed from one front-end channel by switching electrode pairs every 4 ms, dramatically reducing analog circuitry and EMI surface area.
- An oscilloscope's 4-channel input deck uses a CMOS mux on each channel to select between AC, DC, and ground coupling: the mux's three pass-gates correspond to three signal paths (capacitor in series, direct, grounded), letting the user toggle coupling from the front panel without mechanical relays.
A short history
A multiplexer (mux) is a device that selects between several input signals and forwards the selected one to a single output line, with selection directed by separate digital select lines. A multiplexer with 2^n inputs uses n select lines. This lets several input signals share one device or resource, such as one analog-to-digital converter, instead of needing a separate device for each signal.